Laser-induced metallurgical bonding driven without contact

ABSTRACT

A laser, aimed at a flyer plate tab, causes optical energy to be directed at the tab, specifically, at a top surface thereof. Energy impacting the tab accelerates the tab into an impact with a target sheet in a time on the order of about 10 −5  seconds. The impact occurs in excess of 100 m/s, resulting in a metallurgical bond between the tab and the target sheet. The laser preferably strikes the top surface in a normal direction, based upon an initial angularity of the tab relative to the target. The laser emission may be augmented by an ablative layer on the top surface or a transparent covering on the top surface that reacts against the expanding gas from ablative activity on the top surface. The weld is formed without physical contact between the welding device and the tab.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of, and claims benefit of priority to,U.S. patent application Ser. No. 12/921,249, filed 7 Sep. 2010, which isscheduled to issue as U.S. Pat. No. 8,084,710 on 27 Dec. 2011, which isIn turn a national stage entry of PCT/US2009/036499, filed 9 Mar. 2009and which claims the benefit of priority under 35 USC 119 to, U.S.provisional application 61/034,574, filed on 7 Mar. 2008. All of thesecited patent applications are incorporated by reference as if fullyrecited herein.

TECHNICAL FIELD

The present invention is in the field of collision joining.Specifically, the present invention relates to the use of a laser orsome other energy source such as focused non-coherent light or byvaporization of a metal foil by an electrical discharge to impartmomentum to an article to be collision joined.

BACKGROUND OF THE ART

The joining of similar and dissimilar metals is a technical topic thatis of great practical importance and can be quite problematic. This isan issue that is important in a variety of industries such as theautomotive, aerospace, medical, micro-electronics, and food and beverageindustries. Broadly there are two ways that joints can be formed betweenmetals: mechanical interlocks or metallurgical bonds. There are avariety of subcategories of each of these.

Mechanical interlocks can be formed along linear features by variants onfolding metal across itself to create folded seams. Examples of thiskind of joining can be found in the joints that hold can ends on canbodies and automobile closure panels such as doors where an outer panelis interlocked with an inner panel with a fold known as a hem.Mechanical interlocks can also be used at a discrete location. This kindof feature is known as a clinch or clinch-lock and is produced bydeforming two sheets of metal into two interlocking cup-like orreentrant features. This type of interlock is almost universally used tohold the tab onto the end of a beverage can.

Joining between metal components is often accomplished by welding, wherethe defining characteristic is that the two metal surfaces becomeadhered to one another. This is typically done where both metals aremelted (by heat provided in one of numerous ways) until the two metalsbecome mixed (sometimes with a filler metal added) and uponsolidification a joint is formed. Fusion welding has two verysignificant drawbacks. First, it is limited to alloys that makecompatible pairs when mixed. Many alloy systems will form brittleintermetallics when mixed and this provides poor mechanical performance.Second, the heat that is deposited in the weld zone usually dramaticallysoftens the material adjacent to the weld and the melted and solidifiedmaterial itself may be quite weak or brittle. As a result, the metalsthat make up welded structures typically have quite low strength in theregion that makes up the weld. Other related alternatives to fusionwelding also exist. Solid state welding can be accomplished by variantsof resistance or of spot welding, high velocity collision welding (as isusually accomplished using explosives) and ultrasonic welding. Also,interlayers can be used in methods such as brazing and soldering. Thesetechniques can all have significant disadvantages for when consideringspecific applications.

It is well known that if two relatively clean metal surfaces collidewith an appropriate velocity (usually over 200 m/s and typically in therange of from about 150 to 500 m/s) and the impact angle is within aproper range (usually about 15 degrees and typically in the range offrom about 5 to about 25 degrees) the two surfaces may adhere forming ametallurgical joint. This technology is well developed and fairly wellunderstood and practiced in explosive welding and electromagneticwelding. In both these cases a sheet known as a flyer plate is usuallydriven at high speed either by the explosive or electromagnetic forceand strikes a sheet that is often thicker referred to as a target. Uponstriking there is little overall increase in temperature and thematerial is often hardened by the local plastic strain of impact.Similar or dissimilar metals can be joined in this manner as there isnot sufficient time or temperature for the formation of brittleintermetallics. Metallurgical joints formed in this manner can have thestrength of the parent material.

However, explosion welding has some disadvantages, not the least ofwhich are the safety and containment issues associated with the process.Additionally, it is very difficult to couple explosives or magneticpulse energy to small structures (on the order of millimeters orsmaller). However, collision processes also have some tremendousadvantages; most notably the fact that virtually any metal combinationcan be joined using this process. Because the heat required for joiningis localized at the interface and quickly dissipated by the materialsthat are being joined, explosion welds typically do not form large orinterconnected regions of embrittling intermetallic compounds (even incomplex systems) and the residual stresses generated are essentiallynil. For these reasons, the potential for joining materials using highvelocity collisions that do not require the use of explosives offers thepotential for joining otherwise difficult (or impossible) to joinmaterials. These include many of the advanced structural materials basedon intermetallic compounds, composites, nano-structured materials andmetallic glasses.

SUMMARY

The present invention uses intense laser discharges or some other energysource to provide a mechanical impulse to a metal surface by one of avariety of mechanisms. Direct reflection of photons provides some levelof force and impulse. Also, the surface of the metal may ablate underthe beam and this generated gas can also produce a pressure thataccelerates the flyer. The metal surface may also be coated with apolymer or other material that better absorbs optical energy and/or ismore easily ablated. This can generate the same impulse at reduced laserenergy. One additional way to increase the efficiency of converting theoptical energy to mechanical impulse is by placing anoptically-transparent material opposed to the ablated surface to providea surface to oppose the generation of the expanding gas. This will helpto accelerate the flyer plate.

One tremendous advantage of this technique over electromagnetic orexplosive launching is that the shock can be directed to a preciselocation (sub-micron precision) and at a precise time (precision of<10⁻⁵ seconds). The ability to apply enormous pressure at exact andlocalized points on a material interface and to do so with timingaccuracy allows the present invention to create welds in applicationsinvolving micro/nano interfaces.

One particularly attractive product that can be made with thistechnology is the joint between the can tab and can end that is part ofalmost all conventional aluminum and steel beverage cans. Presently thisjoint is created as a mechanical interlock. A protrusion is developed onthe can end using multiple sequential stamping operations by a techniquereferred to as progressive stamping. Once this protrusion (that takes ona shape resembling a cylinder with an open bottom) is created, the tabis placed over it such that the protrusion goes through a hole in thetab and then the protrusion is peened down to capture the tab. Thisrequires significant ductility of the material that makes up the canend. This kind of ductility is generally only found in metals ofrelatively low strength. Because the metal must be relatively weak tomaintain acceptable formability, the can-end metal must be relativelythick to accommodate the structural needs of the can. In principle thetab could also be spot welded to the end, however, this would produce aseverely weakened zone in the proximity of the weld nugget and againthis would necessitate a thicker than optimum weld zone. The presentinvention overcomes the limitations of prior art joining techniquesproviding an ideal solution for placing a tab on a can end as both thetab and end can be produced from thin sections of high strength materialand the as-produced strength of both materials is maintained through thejoining process. Accordingly, the present invention allows for the canend to be fabricated from a thinner piece of higher strength material,thus saving weight and cost in the can and producing associatedenvironmental benefits.

These effects may be realized by a method for producing a spot impactweld between a first part and a second part. The method is conducted byproviding the first and second parts, with a portion of the first partextending bent at an angle out of a generally planar surface of theremainder of the first part. The first and second parts are positionedon a support backing, with the second part between the first part andthe support backing. The first part is positioned so that the secondpart underlies at least the bent portion of the first part with the bentportion bent away from the second part. A laser is aligned to direct itsemitted energy at a top surface of the bent portion. At least one pulseof optical energy is directed from the laser onto the top surface, theamount of energy being sufficient to cause the bent portion tostraighten and impact the underlying second part with a velocity of atleast 100 m/s, resulting in a metallurgical bond between the respectiveparts.

In some aspects, the laser is aligned relative to the bent portion sothat the first of the at least one pulses is substantially normal to thebent portion.

In some aspects, the angle of the bent portion is in the range of fromabout 10 to about 20 degrees.

To augment the inventive effect, many embodiments will comprise the stepof interposing a layer of an ablative material on the top surface of thebent portion prior to the step of directing the at least one opticalenergy pulse.

Another augmentation of the effect may occur from covering the topsurface, or the ablative material on the top surface, if the ablativematerial is present, with a layer of material that is transparent to theoptical energy, but that augments an effect from gas emanating from theablation.

Some aspects of the invention are achieved by a system for producing aspot impact weld of a first part to a second part. Such a systemcomprises a laser; and a means for supporting the first and second partduring a spot welding procedure.

In many of these systems, the laser is a pulse laser capable ofdepositing a directed optical energy beam onto a portion of the firstpart.

In some aspects, the laser is adapted to be aimed to direct the opticalenergy beam therefrom in a line that is substantially normal to a topsurface of the bent portion.

Still further aspects of the invention are achieved by a spot weldedproduct produced by the any one of the above methods.

BRIEF DESCRIPTION OF THE DRAWINGS

Better understanding of the invention, and particularly, the disclosedembodiments thereof, will be had when reference is made to theaccompanying drawings, wherein identical parts are identified withidentical reference numbers and wherein:

FIG. 1 schematically illustrates a system for practicing one embodimentof the present invention; and

FIG. 2 schematically illustrates an article by the system of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In accordance with the foregoing summary, the following presents adetailed description of the preferred embodiments of the invention thatis currently considered to be the best mode.

FIG. 1 depicts a system 100 for the practice of the present invention inaccordance with one embodiment thereof. The system comprises a highpowered laser 102 aimed at a flyer plate 108 positioned on the targetsheet 110 to which the flyer plate tab 108 a will be welded. The targetsheet is supported by a mechanically-stiff back support 112. The angle a114 between the flyer plate tab 108 a and target sheet 110 is about 15degrees, but may fall in the range of from about 5 degrees to about 20degrees. An offset distance without an angle can also permit the twosurfaces to impact at an appropriate angle for impact welding.

The high power pulsed laser 102 is capable of depositing from 0.1 to 100Joules of optical energy focused in a local area on the top surface offlyer plate tab 108 a in approximately 1 microsecond or less. The energyfocused on the flyer plate tab 108 a is accelerated by the interactionof the incident laser beam 104 and top surface of the flyer plate,causing the flyer plate tab 108 a to impact target sheet 110 at avelocity in excess of 100 m/s to thereby develop a metallurgical bondupon impact. The metallurgical bond may have a surface area of from lessthan about 1 mm² to about 100 mm².

The system may be augmented by placing an absorptive and/or ablativelayer 116 on the top surface of the flyer plate tab 108 a and/or byplacing a transparent backing 106 so as to allow it to react against theexpanding gas caused by ablation emanating from the top surface of theflyer plate tab 108 a.

The ablative layer 116 may be formed from a variety of materials thatefficiently ablate when struck with laser beam 104. For example, theablative layer may be a carbon deposit or cellophane-type material. Theablative layer may be shaped (i.e., increasing in thickness in onedirection, having a pyramidal shape, etc.) or may be provided as a filmhaving a near constant thickness.

The transparent backing 106 may be formed of any material through whichthe laser beam 104 may pass without significant loss in optical energyin order to provide sufficient velocity so as to weld the flyer platetab 108 to target sheet 110. Suitable materials include, but are notlimited to, sapphire, quartz, glasses, transparent liquids, such aswater, and polymers.

Acceleration may also be done with some other energy source such asfocused non-coherent light or by vaporization of a metal foil by anelectrical discharge. In either of these cases, a metallurgical bond iscreated through a collision that is effected without a physical contactbetween the device causing the impact and the workpiece beingaccelerated.

Also, it is noted that while the specific embodiment taught herein showsa flyer plate tab which is bent away from an otherwise planar member asthe surface that is being accelerated into a bond-forming collision withanother member, it will be clear to one of skill in this art that therequisite features for practice of the concept described herein are anenergy source that can generate the requisite amount of acceleration byimpacting the surface and a gap between the members being joined. As tothe gap, it is noted that the gap should be sufficiently large to allowthe acceleration to occur, but, at the same instant, be sufficientlysmall to efficiently limit the power needed to effect the acceleration.

FIG. 2 presents the article 200 produced by welding the flyer plate tab108 a of flyer plate 108 to the target sheet 110. A metallurgical bond202 exists between the flyer plate tab 108 a and target sheet 100.

1. A system for producing a metallurgical bond of a flyer plate to atarget sheet, by accelerating a tab of the flyer plate into the targetsheet, comprising: a means for positioning the flyer plate and targetsheet relative to each other, such that the tab is offset from thetarget sheet prior to being accelerated; and a means for inducing theacceleration of the tab into the target sheet without physical contactwith the tab.
 2. The system of claim 1, wherein: the means for inducingthe acceleration is a pulse laser.
 3. The system of claim 2, wherein:the pulse laser is positioned to direct an energy beam therefrom onto atop surface of the tab.
 4. The system of claim 3, wherein: the pulselaser is positioned to initially strike the tab in a substantiallynormal direction.
 5. The system of claim 1, wherein: the tab extendsaway from the target sheet at an acute angle.
 6. The system of claim 1,wherein: the acute angle is in the range of from about 5 to about 20degrees.
 7. The system of claim 6, wherein: the acute angle and thepower of the means for inducing acceleration are selected such that atleast a distal end of the tab strikes the target sheet at an impactvelocity of at least 100 m/s.
 8. The system of claim 7, wherein: thedistal end is accelerated to the impact velocity in a time on the orderof about 10⁻⁵ seconds.
 9. The system of claim 1, wherein: the tabprovides a gap between the flyer plate and the target sheet.
 10. Thesystem of claim 9, wherein: the size of the gap and the power of themeans for inducing acceleration are selected such that at least a distalend of the tab strikes the target plate at an impact velocity of atleast 100 m/s.
 11. The system of claim 10, wherein: the distal end isaccelerated to the impact velocity in a time on the order of about 10⁻⁵seconds.
 12. The system of claim 1, wherein: the means for inducingacceleration causes a distal end of the tab to accelerate to an impactvelocity of at least 100 m/s in a time on the order of about 10⁻⁵seconds.